[PATCHSET sched_ext/for-7.4] sched_ext: Add scx_bpf_cgroup_nr_cpus()

[Andrea Righi <arighi@nvidia.com>]

Hierarchical BPF schedulers need to know how many CPUs a cgroup can run on when
distributing group-wide weights. For example, fair's group share calculation
scales a group's weight by the smaller of its estimated runnable task count and
the number of CPUs in its effective cpuset (tg_cpus() -> cpuset_num_cpus()).
Without that count, a BPF scheduler can only use the machine-wide CPU count and
may give a group confined to a few CPUs excessive shares.

This patchset adds scx_bpf_cgroup_nr_cpus(), a kfunc that returns the same count
fair uses. This allows scx_eevdf to implement proper group scheduling, computing
per-cgroup shares the same way fair does, including for groups confined to a
subset of CPUs through cpuset.

Andrea Righi (3):
      cgroup/cpuset: Protect is_in_v2_mode() in cpuset_num_cpus()
      sched_ext: Introduce scx_bpf_cgroup_nr_cpus()
      selftests/sched_ext: Test scx_bpf_cgroup_nr_cpus()

 kernel/cgroup/cpuset.c                             |   6 +-
 kernel/sched/ext/ext.c                             |  22 ++
 tools/sched_ext/include/scx/common.bpf.h           |   1 +
 tools/sched_ext/include/scx/compat.bpf.h           |   7 +
 tools/testing/selftests/sched_ext/Makefile         |   6 +-
 .../selftests/sched_ext/cgroup_nr_cpus.bpf.c       |  60 +++
 tools/testing/selftests/sched_ext/cgroup_nr_cpus.c | 419 +++++++++++++++++++++
 tools/testing/selftests/sched_ext/config           |   1 +
 8 files changed, 519 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/cgroup_nr_cpus.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/cgroup_nr_cpus.c